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Truing my 4 Jaw Chuck

Tom Griffin

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I think the term independent is key here. Unlike three jaw or six jaw universal chucks, the jaws in a four jaw chuck are meant to be independent of each other, so there is no need to match them to each other. Also, the purpose of the 45 degree cuts on the side of the jaws is to provide clearance around the work piece. There is no need to true them up, except maybe for aesthetics. I clean mine up once in a while on the belt sander to make them look better. In the case of a universal chuck, if the inside of jaws has become worn they can be reground, but first they need to be clamped on the inside of an accurately bored ring to pre-load them. This will remove any play between the jaws and the scroll plate so they don't move around during the grinding process. It should be noted though that the scroll will wear as well so even re-ground jaws may not run true at all diameters.

Any radial runout of the chuck body can be corrected by loosening the chuck mounting screws and indicating the chuck body on the backing plate. Grinding the radial surface of the body is not really necessary except for aesthetics as with the clearance cuts on the sides of the jaws. What is necessary is getting the backing plate to run true axially. That can be accomplished by grinding or turning the mounting face as you did. It is doubtful that the mounting face on the chuck body itself was not true. Those are normally Blanchard ground at the factory so the front an rear faces are parallel. You may want to remove the jaws and run an indicator on the front face of the chuck to be sure it still runs true after you ground the back face.

Good job figuring out all the different set-ups, especially mounting the chuck backwards on its own backing plate. Figuring out good set-ups with the equipment available is what machining is all about.

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I'm confused also as to why you went to that trouble on a 4 jaw. If a piece was in the 4 jaw as you say, and was making a circle around the tailstock, then that just says that the piece in the 4 jaw wasn't centered. As far as the 45 degree angles, they do not have to be uniform or centered on the jaw itself. One side could be 45 and the other side could be 48 degrees. All that angle is for, is for clearance when the jaws close up, so you could put a smaller piece of stock in. As far as the outside of the chuck running true, that doesn't matter at all. If you are getting vibration with it being .005 out, then you are having some other issues.

A three Jaw chuck is completely different. You have one screw turning all three jaws at once. They need to be true. A four jaw....each jaw is independent of one another and each has to be adjusted accordingly depending on the stock you have in the chuck.

Unless I am really missing something, you didn't need to do anything to the 4 jaw at all.


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I think I understand what turbine was trying to fix... Evidently the clamping faces of his jaws weren't aligned parallel (square radially) to the axis of his lathe. This would make any perfectly centered workpiece describe a cone shape in space... In other words, even if he dialed in zero TIR in the workpiece at the chuck, the far end would be out of kilter.

I would think the chuck's back side or the mounting plate is out of whack, not the jaws. I'm also not sure why the 45's should have anything to do with the problem.



John, That was my take on Turbine's plight also.

Tom, "This will remove any play between the jaws and the scroll plate so they don't move around during the grinding process. It should be noted though that the scroll will wear as well so even re-ground jaws may not run true at all diameters. " We're talking about a four jaw here, there's no scroll.

Turbine, congrats! although I, and others, may have gone about it differently I think you did a good job and a good job of your documentation.


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I have to make a confession first. I love to look at other peoples pictures of there projects but don't take the time to get pictures of my own. I started this project with the intention of lots of pictures and details but while working with the little time that I get in the shop, I find it hard to grab the camera and take the pictures. While I did get some pictures there are more that I should have taken.

Next I have to confess again that I am a newbie to machining and am doing this project the way I think is best at the time. I know and hope that those of you with more experiance with this type of project will chime in with "that was the correct way to do that step" and "that step should or could have been done better this way". All in all I am open to constructive critism and welcome it, I listen to what other say and weed out what I think to be useful to me, so please feel free to add any comments that you feel are needed.

The first time I mounted my 4 jaw chuck and turned on the machine really took me by suprise. The chuck was so far out of balance that the whole machine was shaking. It wasn't shaking bad enough that I didn't turn the piece that I had to turn but bad enough that that was the only piece that I turned. While turning this piece without the tail stock, I noticed a really bad chatter. That turned out to be from the jaws holding the work piece on the inside part of the jaws but not the out side of the jaws. We all know that that means that the jaws are not true, so I started to grind them to true them up.

View attachment 32142

Using my Dremel as a tool post grinder and using the jaws on either side to hold the jaw I was grinding on in place, I started to grind straight in and out on the jaw faces. Later after reading in here about truing a three jaw chuck I realized that I needed to have the chuck turning to get the proper radus on the jaw faces. I also found that the 45 degree angles on either side of the jaws were not ground the same from the factory. They were 45 degree angles but one was ground in further on one side than the other which meant that the center surface was not truly centered.

More to come.

I see quite a few things wrong with how you went about this. Don't take it as anything but constructive criticism. If you were working in a machine shop and this was a $10,000 piece of equipment, you would have been called on the carpet.

I understand when you say that when you chucked up a piece that out towards the tailstock, it was running out of round or cone shaped. How did you first determine that it was the jaws that was causing it? Did you take each individual jaw itself out and check it for square? In the above picture you show that you are grinding the jaw face itself. Yet you are using a wheel on a 1/8" shaft. A chuck jaw is hard. How do you know that the shaft of your grinding wheel didn't flex the further back you went? Then you show that you turned the chuck around, used the chucks jaws, and clamped it to what appears to be the mounting plate on the lathe to try and true up the back of the chuck itself. From the factory, the front and back of the chuck are ground parallel already, so no truing of that should have been necessary. Secondly, you didn't check the jaw for squareness. Then you machined something, then used that portion to machine something else, by that meaning machining the jaws, then using them to machine the back of the chuck. So you have now created multiple places for error.

What should have been done if you had any doubt about the chuck, would be to set it up on a surface plate, on 1-2-3 blocks if you have pins in the back, put in a known piece of straight stock, then checked that piece of straight stock for squareness. You could have used Prussian Blue on your square stock to see if the jaws were hitting it correctly and the length of the jaw.

You always want to check everything first and eliminate the possibilities before just jumping in, throwing a piece of stock in, and calling it bad. Did you even check the stock you had in it for out of roundness or for a possible bend in it?

Like I said, just some constructive criticism, but next time, research the situation a little further. In your tools and gages, keep a 1" drill blank, and a tube of Prussian Blue. These two things will go a long way in inspecting parts or fixturing. Also if you don't have a good precision square, I would recommend that you get one. Or get a set of good precision squares like 4" 6" 8" and a 12". They all come in very handy.

And again, don't take it the wrong way with what I said, the ultimate thing is that you are happy. But in a shop where you are working with multi thousand dollars worth of product, the same practice goes on a product as it does the machine that makes it. Where work, I have to work in tolerances of anywhere from .005 down to .0005 and less depending on what it is, so one has to learn to be thick skinned and critical at the same time.

But the ultimate goal is that you are happy, everything worked out, and now lets see some parts made!!!:biggrin:


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When you indicate a piece you indicate as close to the chuck as possible then you run the indicator down to the end,if it is out and most of the time it is you use a brass hammer and bump it in ,repeat till both end are running true. Even with a 3 jaw you have to do this if it isn't perfect. I have ran 4 jaws that are sprung from somebody tighting with a cheater bar,that will screw up a chuck.


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I can understand trueing up the jaws if they are sprung, or belled. I have trued my 4-jaw chucks from time to time. I have found my cheapo china chucks dont grip with the full jaw. I have run across that when bought new. Regrind time!

Though many folks elect to bump the jaw with a hammer, Id rather regrind the jaws true so they can hang on to the worlk piece properly. Any time I notice the jaws are belled out, 3, or 4 jaw, thats when I get the tool post grinder out, and true things up. Must addmit, Ive never had to cut the angles though. By the sounds of the OP, he just wanted to do this to correct a factory mistake .

Many machinist grip the work with the outer portion of the jaws, and over time , and over tightening will bell out. I know some will say not to swallow the work piece to full depth, but hey, sometimes you have to, or need to.